THE ABSENCE OF CHARGE BACKSCATTERING IN THE NONEQUILIBRIUM CURRENT OFNORMAL-SUPERCONDUCTOR STRUCTURES

We study the nonequilibrium transport properties of a normal-supercond uctor-normal structure, focusing on the effect of adding an impurity i n the superconducting region. Current conservation requires the superf luid velocity to be nonzero, causing a distortion of the quasiparticle dispersion relation within the superconductor. For weakly reflecting interfaces we find a regime of intermediate voltages in which Andreev transmission is the only permitted mechanism for quasiparticles to ent er the superconductor. Impurities in the superconductor can only cause Andreev reflection of these quasiparticles and thus cannot degrade th e current. At higher voltages, a state of gapless superconductivity de velops which is sensitive to the presence of impurities.